Anaplasma phagocytophilum is a tick-borne pathogen responsible for granulocytic anaplasmosis in humans (Bakken J. S., et al.: Human granulocytic ehrlichiosis in the upper Midwest United States. A new species emerging? JAMA 272: 212-218, 1994). There has been a steady rise in cases of Anaplasma infections, alone or through co-infection with other tick-borne pathogens (Varde S., et al.: Prevalence of tick-borne pathogens in Ixodes scapularis in a rural New Jersey County. Emerg. Infect. Dis. 4: 97-99, 1998). Left unchecked, Anaplasma infection can be a potentially fatal disease resulting from the targeting and replication of the Anaplasma pathogen within human neutrophils (Bakken J. S. et al.: JAMA 272: 212-218, 1994). Anaplasma phagocytophilum infection thus emerges as a significant healthcare concern.
Detection of Anaplasma infection is crucial. Ideally, a diagnostic assay should be capable of detecting Anaplasma infection at its early stages, when antibiotic treatment is most effective and beneficial. Traditional detection methods for Anaplasma infection includes: (i) microscopic identification of morulae in granulocytes, (ii) PCR analysis using whole blood, (iii) isolation of the Anaplasma bacterium from whole blood, and (iv) serological tests, particularly indirect immunofluorescence assay (IFA). Microscopic examination is tedious and prone to sampling error. PCR test is sensitive in detecting the tick-borne pathogen during the period of time when the pathogen is present in the blood of infected patients. IFA is most commonly used (Park, J., et al.: Detection of antibodies to Anaplasma phagocytophilum and Ehrlichia chaffeensis antigens in sera of Korean patients by western immunoblotting and indirect immunofluorescence assays. Clinical and Diagnostic Laboratory Immunology 10(6): 1059-1064, 2003), but this test often gives false positive results. Such results can be attributed in part to the use of whole-cell antigens because such proteins may be shared with other bacteria (Magnarelli, L. A., et al.: Use of recombinant antigens of Borrelia burgdorferi and Anaplasma phagocytophilum in enzyme-linked immunosorbent assays to detect antibodies in white-tailed deer. J. Wildlife Dis. 40(2): 249-258, 2004). When clinical symptoms are manifested or high and stable antibody titers to Anaplasma phagocytophilum are found in patient blood, it reaches a late infection stage and bypasses the window of antibiotic treatment.
So far, there are only a few surface proteins on Anaplasma pathogen that are used in diagnostic assay for immuno-responses (i.e., IgG and IgM responses). It is generally believed that outer membrane proteins in pathogens are target for eliciting an immuno-response because they may be the first to be exposed to immune cells of a host. Regarding the Anaplasma phagocytophilum species, U.S. Pat. No. 6,964,855 discloses the use of an outer membrane protein and its fragments in a detection assay. U.S. Pat. No. 7,304,139 discloses a major surface protein 5 (MSP5) and its use in a diagnostic test. The '139 patent discloses a few patient's reactivity towards MSP5 and it lacks any data relating sensitivity and specificity, let alone any IgG/IgM distinction. Zhi et al. discloses cloning and expression of an outer membrane protein of 44 kDa and its use in a Western immunoblot assay (J. Clinical Microbiology 36(6): 1666-1673, 1998). Both MSP5 and p44 are outer membrane proteins in Anaplasma phagocytophilum. 
The host immune response to tick-borne pathogen infection is frequently vigorous, and it is typically an easy task for investigators to identify many antigens (e.g., outer membrane proteins) which have been targeted as part of the host antibody response to infection. However, most of these antigens often fail when used as biomarkers for diagnostic purposes. Hence, it is well established that there is often no correlation between protein antigenicity, to the extent such a parameter can even be accurately predicted, and whether or not a given protein might serve as a useful diagnostic marker. It is the present inventors' contention that the proteins most beneficial as biomarkers for infection, and useful for assay development, are those that manage to evade the host immune response; successful identification of these specific antigens is a largely unmet challenge in the diagnostics industry, despite urgent needs for such biomarkers.
There remains a continuing need in the discovery of a novel antigen present in Anaplasma phagocytophilum that is useful and can provide a highly specific and sensitive test for sero-detection of this pathogen.